Issue: Allow extensions to control their own palettes

In previous versions (up to at least 3.2, I believe) extension blocks were put in the palette in the same order that they appeared in blocks.xml. Now it appears they are ordered almost at random. It would be great to be able to order them ourselves again. Perhaps this could be even taken a step further, giving each extension complete control over its own palette. (I think I've had a discussion with someone about this but I can't remember...)